This invention relates to slow speed tape recorders and in particular to those magnetic tape recorders which are utilized in the recording of physiological signals of a patient.
In conjunction with the examination of a patient to determine general cardiac conditions and the diagnosing of health problems relating thereto, electrical signals that circulate upon the surface of a person's skin as a result of expansions and contractions of the cardiac muscles are recorded on magnetic tape and subsequently analyzed by a cardiologist or other trained medical personnel. In the recording of such signals, a portable-type tape recorder may be used to obtain the continous ECG signals for a long period of time, such as 24 hours or so. In one device for this purpose, the Model 7007 Recorder supplied by the American Optical Corporation, the recording is done on a tape cassette somewhat similar to standard cassettes utilized in a portable tape recorder. In order to accomplish a recording for a period of such as 24 hours on a single "standard" tape cassette, the tape speed must be well below the conventional 17/8 inches per second. Accordingly, the above-mentioned device utilized a tape speed of approximately 1/16 inch-per-second to achieve this long duration recording.
It should be recognized that it is very important that the tape speed be constant throughout the duration of the recording, so that when the tape is reviewed, the time intervals between the particular portions of the ECG signal which are of interest (e.g. the R-wave) that an accurate record is available. For instance, the distance between R-waves or the time interval therebetween indicating successive beats of the heart are often measured. It will be noted that these time intervals might vary greatly if the tape speed were not held constant during a recording and playback thereof. Thus, it is necessary to keep both the high frequency variation (flutter) and the low frequency variation (wow) to a minimum. As may be recognized, this can be exceedingly difficult in a miniature portable slow speed tape recorder.
The recorder of the above design, in order to accomplish the desired ends, utilizes a battery-powered drive including a synchronous AC motor. The power to the synchronous motor is provided by converting the constant DC current available from the battery to a square wave DC signal analogous to an AC signal. Additionally, this signal is divided and one portion thereof is phase-shifted so that a split-phase, or two-phase, current source is available to drive the corresponding phases of the AC synchronous motor.
An AC motor nominally rated at 300 rpm at 60 cycles AC at 12 volts and 0.75 oz/in. of torque has been found to be quite effective as a capstan drive in the portable tape recorder in the above-identified unit. The motor, it should be recognized, is not operated at rated conditions, however, being driven at 4 volts switched plus and minus at a frequency of 64 hertz. It should be recognized by those skilled in the art that 64 is a convenient multiple of conventional clocking devices. The motor has been found to have sufficient torque for the driving of the capstan and the related cassette to provide very stable tape operation, particularly when the unit is incorporated with such features as is described in U.S. Pat. Nos. 3,882,543; 3,913,869; and 3,982,277, all assigned to the assignee of the present application.
The drive mechanism illustrated in U.S. Pat. No. 3,913,869 particularly enables the motor of the present recorder to reach operating speed synchronously while picking up the load of the capstan and the cassette. This is accomplished through the clutch mechanism and the belt arrangement disclosed in the afore-mentioned patent which allows the synchronous motor sufficient revolution to develop the torque necessary to drive the related load. While this illustrated drag mechanism is sufficient to enable the synchronous recorder motor to reach operating speed and assume the load of the cassette under normal operating conditions, it is essential in the initial moments of operating such a physiological recorder that continuous synchronous operation be assured. During these initial first moments of operation of the recorder, calibrations are performed and the coded timing track of the ECG recording tape is initiated. In order to assure that the subsequent long duration 24-hour recording begins from a properly calibrated base, rated speed operation of the motor is essential. In view of this desired high reliability of operation, further startup stabilization of the synchronous motor was felt important.
By providing special startup operating mode for the recorder, it is also further felt that economics in current usage under continued operation might be gained. It is well recognized that the maximum torque of a synchronous motor is achieved during the synchronous operation. Thus, the present invention, directed to assuring a fully synchronous operation of a battery-powered synchronous motor throughout the full operating range thereof (including startup) promotes efficient use of the battery power. Particularly, we have determined that a special operating mode for the first few moments of recorder operation additionally enhances the calibration of the tape operation and thus expedites the application of the recorder to patient ECG signal monitoring.
These and further objectives of the present invention will be understood from the following description of the invention.